Code division multiple access (CDMA) communication systems provide wireless communications between a base station and one or more mobile or portable subscriber units. The base station is typically a computer-controlled set of transceivers that are interconnected to a land-based public switched telephone network (PSTN). The base station further includes an antenna apparatus for sending forward link radio frequency signals to the mobile subscriber units and for receiving reverse link radio frequency signals transmitted from each mobile unit. Each mobile subscriber unit also contains an antenna apparatus for the reception of the forward link signals and for the transmission of the reverse link signals. A typical mobile subscriber unit is a digital cellular telephone handset or a personal computer coupled to a cellular modem. In such systems, multiple mobile subscriber units may transmit and receive signals on the same center frequency, but different modulation codes are used to distinguish the signals sent to or received from individual subscriber units.
In addition to CDMA, other wireless access techniques employed for communications between a base station and one or more portable or mobile units include time division multiple access (TDMA), the global system for mobile communications (GSM), the various 802.11 standards described by the Institute of Electrical and Electronics Engineers (IEEE) and the so-called “Bluetooth” industry-developed standard. All such wireless communications techniques require the use of an antenna at both the receiving and transmitting end. Any of these wireless communications techniques, as well as others known in the art, can employ one or more antennas constructed according to the teachings of the present invention. Increased antenna gain, as taught by the present invention, will provide improved performance for all wireless systems.
The most common type of antenna for transmitting and receiving signals at a mobile subscriber unit is a monopole or omnidirectional antenna. This antenna consists of a single wire or antenna element that is coupled to a transceiver within the subscriber unit. The transceiver receives reverse link audio or data for transmission from the subscriber unit and modulates the signals onto a carrier signal at a specific frequency and modulation code (i.e., in a CDMA system) assigned to that subscriber unit. The modulated carrier signal is transmitted by the antenna. Forward link signals received by the antenna element at a specific frequency are demodulated by the transceiver and supplied to processing circuitry within the subscriber unit.
The signal transmitted from a monopole antenna is omnidirectional in nature. That is, the signal is sent with approximately the same signal strength in all directions in a generally horizontal plane. Reception of a signal with a monopole antenna element is likewise omnidirectional. A monopole antenna alone cannot differentiate a signal received in one azimuth direction from the same or a different signal coming from another azimuth direction. Also, a monopole antenna does not produce significant radiation in the zenith direction. The antenna pattern is commonly referred to as a donut shape with the antenna element located at the center of the donut hole.
A second type of antenna that may be used by mobile subscriber units is described in U.S. Pat. No. 5,617,102. The system described therein provides a directional antenna system comprising two antenna elements mounted on the outer case of a laptop computer, for example. The system includes a phase shifter attached to each element. The phase shifters impart a phase angle delay to the signal input thereto, thereby modifying the antenna pattern (which applies to both the receive and transmit modes) to provide a concentrated signal or beam in a selected direction. Concentrating the beam is referred to as an increase in antenna gain or directivity. The dual element antenna of the cited patent thereby directs the transmitted signal into predetermined sectors or directions to accommodate for changes in orientation of the subscriber unit relative to the base station, thereby minimizing signal losses due to the orientation change. The antenna receive characteristics are similarly effected by the use of the phase shifters.
CDMA cellular systems are recognized as interference limited systems. That is, as more mobile or portable subscriber units become active in a cell and in adjacent cells, frequency interference increases and thus bit error rates also increase. To maintain signal and system integrity in the face of increasing error rates, the system operator decreases the maximum data rate allowable for one or more users, or decreases the number of active subscriber units, which thereby clears the airwaves of potential interference. For instance, to increase the maximum available data rate by a factor of two, the number of active mobile subscriber units can be decreased by one half. However, this technique is not typically employed to increase data rates due to the lack of priority assignments for individual system users. Finally, it is also possible to avert excessive interference by using directive antennas at both (or either) the base station and the portable units.
Generally, a directive antenna beam pattern can be achieved through the use of a phased array antenna. The phased array is electronically scanned or steered to the desired direction by controlling the phase of the input signal to each of the phased array antenna elements. However, antennas constructed according to these techniques suffer decreased efficiency and gain as the element spacing becomes electrically small compared to the wavelength of the transmitted or received signal. When such an antenna is used in conjunction with a portable or mobile subscriber unit, the antenna array spacing is relatively small and thus antenna performance is correspondingly compromised.
Various disadvantages are inherent in prior art antennas used on mobile subscriber units in wireless communications systems. One such problem is called multipath fading. In multipath fading, a radio frequency signal transmitted from a sender (either a base station or mobile subscriber unit) may encounter interference in route to the intended receiver. The signal may, for example, be reflected from objects, such as buildings, thereby directing a reflected version of the original signal to the receiver. In such instances, the receiver receives two versions of the same radio signal; the original version and a reflected version. Each received signal is at the same frequency, but the reflected signal may be out of phase with the original signal due to the reflection and consequent differential transmission path length to the receiver. As a result, the original and reflected signals may partially or completely cancel each other (destructive interference), resulting in fading or dropouts in the received signal, hence the term multipath fading.
Single element antennas are highly susceptible to multipath fading. A single element antenna has no way of determining the direction from which a transmitted signal is sent and therefore cannot be turned to more accurately detect and receive a signal in any particular direction. Its directional pattern is fixed by the physical structure of the antenna. Only the antenna physical position or orientation (e.g., horizontal or vertical) can be changed in an effort to obviate the multipath fading effects.
The dual element antenna described in the aforementioned reference is also susceptible to multipath fading due to the symmetrical and opposing nature of the hemispherical lobes formed by the antenna pattern when the phase shifter is activated. Since the lobes created in the antenna pattern are more or less symmetrical and opposite from one another, a signal reflected toward the backside of the antenna (relative to a signal originating at the front side) can be received with as much power as the original signal that is received directly. That is, if the original signal reflects from an object beyond or behind the intended receiver (with respect to the sender) and reflects back at the intended receiver from the opposite direction as the directly received signal, a phase difference in the two signals creates destructive interference due to multipath fading.
Another problem present in cellular communication systems is inter-cell signal interference. Most cellular systems are divided into individual cells, with each cell having a base station located at its center. The placement of each base station is arranged such that neighboring base stations are located at approximately sixty-degree intervals from each other. Each cell may be viewed as a six-sided polygon with a base station at the center. The edges of each cell abut and a group of cells form a honeycomb-like image if each cell edge were to be drawn as a line and all cells were viewed from above. The distance from the edge of a cell to its base station is typically driven by the minimum power required to transmit an acceptable signal from a mobile subscriber unit located near the edge of the cell to that cell's base station (i.e., the power required to transmit an acceptable signal a distance equal to the radius of one cell).
Intercell interference occurs when a mobile subscriber unit near the edge of one cell transmits a signal that crosses over the edge into a neighboring cell and interferes with communications taking place within the neighboring cell. Typically, signals in neighboring cells on the same or closely spaced frequencies cause intercell interference. The problem of intercell interference is compounded by the fact that subscriber units near the edges of a cell typically employ higher transmit powers so that their transmitted signals can be effectively received by the intended base station located at the cell center. Also, the signal from another mobile subscriber unit located beyond or behind the intended receiver may arrive at the base station at the same power level, causing additional interference.
The intercell interference problem is exacerbated in CDMA systems, since the subscriber units in adjacent cells typically transmit on the same carrier or center frequency. For example, generally, two subscriber units in adjacent cells operating at the same carrier frequency but transmitting to different base stations interfere with each other if both signals are received at one of the base stations. One signal appears as noise relative to the other. The degree of interference and the receiver's ability to detect and demodulate the intended signal is also influenced by the power level at which the subscriber units are operating. If one of the subscriber units is situated at the edge of a cell, it transmits at a higher power level, relative to other units within its cell and the adjacent cell, to reach the intended base station. But, its signal is also received by the unintended base station, i.e., the base station in the adjacent cell. Depending on the relative power level of two same-carrier frequency signals received at the unintended base station, it may not be able to properly differentiate a signal transmitted from within its cell from the signal transmitted from the adjacent cell. There is required a mechanism for reducing the subscriber unit antenna's apparent field of view, which can have a marked effect on the operation of the forward link (base to subscriber) by reducing the number of interfering transmissions received at a base station. A similar improvement in the reverse link antenna pattern allows a reduction in the desired transmitted signal power, to achieve a receive signal quality.